1. Field of the Invention
The present invention relates to an electrostatic capacity type encoder, and more particularly to an improvement of the electrostatic capacity type encoder which electrically detects relative rotary displacement of a rotary disk.
2. Prior Art
In the prior art device exits a measuring apparatus using an encoder which converts the displacement of a probe moving on the apparatus itself and being brought in contact with an object to be measured into a electric signal output pulse to be counted by a counter, for example, so that a counter value can be displayed on the display instrument as a digital representation of measured value.
As for the encoder to be used for this kind of measuring apparatus, there has been used an electrostatic capacity type encoder wherein at least a pair of electrode plates are arranged to face each other to form a capacitor. Both of the electric plates are relatively moved in accordance with the displacement of the probe, and the mechanical displacement is electrically detected as a change in electrostatic capacity of the capacitor.
In such encoder, however, since a tap circuit (voltage dividing circuit) is formed by the capacitor consisting of the relatively moving electrodes and the displacement of the probe is detected on the basis of tap ratio which changes in accordance with the electrostatic capacity of the capacitor, the tap output does not correspond and follow the displacement of the probe with accuracy so that the measurement of the probe cannot be performed with high accuracy, when the changes in distace between the moving electrode plates forming the capacitor fluctuates to change the electrostatic capacity, and when the voltage of power supply changes which is applied to the tap circuit.
In the prior art device, furthermore, an electrostatic capacity type encoder is offered to solve such disadvantages as mentioned above. In this encoder a plurality of transmitting electrodes are arranged with an equal distance therebetween and applied there to is an alternating current having predetermined differed phases, and on the opposide side of these transmitting electrodes is provided the receiving electrodes. In this encoder the relative displacement between the transmitting electrode and the receiving electrode is detected through the receiving electrode so that the electrostatic capacity signal period can be obtained therefrom.
In FIGS. 6 and 7 shown therein is such an electrostatic capacity type encoder in the prior as is mentioned above. This encoder includes a disk plate 10 fixed to a rotary shaft 10a rotatingly attached to the body, and a stationary disk arranged on the body to face the rotary disk 10 in order to detect the amount of the rotating displacement in the rotary disk 10 relative to the stationary disk 12.
Accordingly, on the surface of the stationery disk 12 is provided a plurality of transmitting electrodes 14 at an equal distance therebetween along the circumference direction. To these electrodes 14 a voltage applying circuit 16 is connected to apply the alternating current of sinusoidal wave or rectangular wave shape one after another with differed phases by a predetermined value, for example .pi./4 in the embodiment, and plural units of electrode group 100 are formed with 8 phase electrodes as one unit.
On the other hand, on the surface of the rotary disk 10 is provided the same number of the receiving electrode 18 with the units of electrode group 100 so that they can face to a series of predetermined transmitting electrodes 14 included in the respective units of the electrode group 100. In the embodiment shown in FIG. 7, the receiving electrode 18 is facingly arranged to span four consecutive transmitting electrode 14, that is, the transmitting electrode which a standard V.sub.1 is applied thereto and the transmitting electrodes to which the respective voltages V.sub.2, V.sub.3 and V.sub.4 are applied with the phases differed by .pi./4, 2.pi./4 and 3.pi./4 from the standard voltage V.sub.1.
Furthermore, on the surface of the rotary disk 10, a ground electrode 20 is provided between the respective receiving electrodes 18 at a position facing the transmitting electrode 14 to which applied are the voltage with the phases differed by 4.pi./4, 5.pi./4, 6.pi./4 and 7.pi./4 from the standard voltage V.sub.1 to prevent interference with the electrostatic capacity for occurring between the respective receiving electrodes 18 and other sources.
In the composition described above, when the rotary disk 10 is rotated, an electrostatic capacity signal is detected with a periodical change in accordance with the amount of rotating displacement in the rotary disk 10 from the receiving electrodes 18 by means of relative movement between the transmitting electrodes 14 and the receiving electrodes 18.
FIG. 8 is a sham illustrating rectilinearly expanding the relative position among the transmitting electrodes 14, the receiving electrodes 18 and the ground electrodes 20 in the encoder shown in FIGS. 6 and 7. FIG. 9 shows vector illustrations of various periodical changes in the electrostatic capacity signal obtained through the receiving electrodes 18.
The detected voltage V.sub.0 of the electrostatic capacity signal corresponds to the composite voltage vector of the respective transmitting electrodes 14 which the receiving electrodes 18 face. Accordingly, the amount of rotating displacemeent in the rotary disk 10, in other words, the relative displacement between the transmitting electrodes 14 and the receiving electrodes 18, can be obtained by detection of the phase difference in the composite vector V.sub.0 from the standard voltage V.sub.1.
In other words, each of the transmitting electrodes 14 has a width l and the receiving electrodes 18 are respectively formed having a width of 4l. When the position of the rotary disk 10 to the stationary disk 12 is in the state shown in FIG. 8, the voltage V.sub.0 is outputted in accordance with the addition of the voltage vectors V.sub.1, V.sub.2, V.sub.3 and V.sub.4 applied from the receiving electrodes 18 to the respective transmitting electrodes 14, and the o of V.sub.0 relative to V.sub.1 is detected as the phase difference as is shown in FIG. 9(a).
In the next stage, when the rotary disk 10 rotates and the receiving electrodes 18 move by l/2 with respect to the transmitting electrode 14, V.sub.0 is detected to be advanced by .phi.=90 degrees in comparison with V.sub.1, as is shown in FIG. 9(b). When the rotary disk 10 further rotates so that the receiving electrodes 18 move by l from the initial stage, the phase of the electrostatic capacity signal advances as shown in FIG. 9(c) in the same manner. When the receiving electrodes 18 move by 2l, 3l, 4l, 5l and 6l, the phases of the electrostatic capacity signal advance as shown in FIGS. 9(d)-9(h) in comparison with V.sub.1.
Accordingly, detection of the phase difference .phi. from the standard voltage V.sub.1 in the electrostatic capacity signal provides the displacement of the receiving electrodes 18 relative to the transmitting electrodes, that is, the amount of rotating displacement in the rotary disk 10 relative to the stationary disk 12.
However, there is a problem in the way of picking up the electrostatic capacity signal which is produced from the receiving electrodes 18 from the side of the rotary disk 10.
In the encoder in the prior art device, as shown in FIG. 7, an output electrode 22 in a ring shape is provided in the inside of the transmitting electrodes 14 on the surface of the stationary disk 12. The receiving electrodes 18 provided on the rotary disk 10 are formed to face the output electrode 22 provided on the stationary disk 12 to couple both of the electrodes 18 and 22 electrically.
According to the composition mentioned above, this encoder is capable of outputting the electrostatic capacity signal shown in FIG. 9 obtained from the receiving electrodes 18 by electrostatic coupling without using mechanical contact between the rotary disk 10 and the stationary disk 12.
Therefore, according to the conventional electrostatic capacity type encoder, the displacement in the rotary disk 10 can be measured with simple composition and high accuracy.
In the recent years, however, it has been requested to design the electrostatic capacity type encoder to be smaller in size and lighter in weight without decreasing the accuracy in order to increase portability and operational capabilities of the device.
In order to obtain preferable accuracy in the electrostatic capacity type encoder, it is necessary to establish suitable electrode dimensions and intervals between the respective electrodes, specifically, the transmitting electrodes 14 in accordance with the required resolution.
In the electrostatic capacity type encoder in the prior art device, however, since both the transmitting electrodes 14 and the output electrode 22 are provided together on the stationary disk 12, it is necessary to enlarge the dimensions of the rotary disk 10 and its facing stationaery disk 12 in order to increase the accuracy. Consequently, there is such a disadvantage that the device cannot be designed to be smaller in size in the radial direction.
In the conventional electrostatic capacity type encoder, since the transmitting electrodes 14 and the output electrode 22 are adjacently arranged on the same stationary disk 12, interference with the electrostatic capacity results from interaction which occurs between the electrodes 14 and 22, and therefore, the output signal includes some noise.
In order to solve the problem mentioned above, it is necessary to establish a suitable distance between the transmitting electrodes 14 and the output electrode 22 and to provide a ground electrode 24 in the ring shape between the electrodes 14 and 22. Consequently, it is inevitable that the stationary disk 12 becomes larger and more complicated in its shape, and there is a drawback in that the device cannot be designed small enough.
Furthermore, in this kind of electrostatic capacity type encoder, the transmitting electrodes 14 and the output electrode 22 are provided on the stationary disk 12, and electric circuits 14a and 22a of the respective electrodes 14 and 22 must be adjacently arranged in a complicated manner on the back side of the stationary disk 12, as is shown in FIG. 10. Consequently, both of the circuits 14a and 22a interfere with each other, and it is inevitably disadvantageous that the noise therefrom decreases the accuracy. As the voltage applying circuit 16 and the signal detector 36 to be connected with the transmitting electrode 14 and the output electrode 22 must be placed at a certain distance apart so as not to pick up noise from each other, more space is required to place these electric circuits, which become a problem in minimizing the total size of the device.